Roll out the puff pastry on a lightly floured surface into a 12-by-16-inch rectangle. Cut in half crosswise to make two 8-by-12-inch rectangles, then cut each piece crosswise into thirds to make six 4-by-8-inch rectangles. Cut each rectangle in half diagonally to make 12 triangles.
Line a baking sheet with parchment paper. Starting from the short side, loosely roll up each pastry triangle; place seam-side down on the prepared baking sheet, about 1 1/2 inches apart. Refrigerate until firm, about 30 minutes.
Preheat the oven to 350 degrees F. Whisk the egg yolk with 1 teaspoon water; brush on the pastry. Bake until puffed and golden brown, 30 to 35 minutes. Reduce the oven temperature to 300 degrees F and continue baking until the croissants are cooked through, 35 to 40 more minutes. Let cool completely.
Make the meringue: Whisk the egg whites, sugar, lemon juice and salt in a large heatproof bowl until smooth. Place the bowl over a saucepan with 1 inch of simmering water (do not let the bottom of the bowl touch the water); cook, whisking, until the mixture is warm and the sugar is dissolved, 2 to 3 minutes. Remove the bowl from the pan and beat with a mixer on medium-high speed until the meringue is cool and stiff glossy peaks form, 5 to 10 minutes.
Poke a hole in the bottom of each croissant with a thick skewer or chopstick, wiggling it around to create a small pocket. Fill a resealable plastic bag with the lemon curd; snip a corner. Pipe about 1 tablespoon curd into each croissant. Swirl the meringue on top of each croissant using the back of a spoon. Lightly toast the meringue with a kitchen torch.